Selective emitter using a screen printed etch barrier in crystalline silicon solar cell
© Song et al.; licensee Springer. 2012
Received: 20 April 2012
Accepted: 11 July 2012
Published: 23 July 2012
The low level doping of a selective emitter by etch back is an easy and low cost process to obtain a better blue response from a solar cell. This work suggests that the contact resistance of the selective emitter can be controlled by wet etching with the commercial acid barrier paste that is commonly applied in screen printing. Wet etching conditions such as acid barrier curing time, etchant concentration, and etching time have been optimized for the process, which is controllable as well as fast. The acid barrier formed by screen printing was etched with HF and HNO3 (1:200) solution for 15 s, resulting in high sheet contact resistance of 90 Ω/sq. Doping concentrations of the electrode contact portion were 2 × 1021 cm−3 in the low sheet resistance (Rs) region and 7 × 1019 cm−3 in the high Rs region. Solar cells of 12.5 × 12.5 cm2 in dimensions with a wet etch back selective emitter Jsc of 37 mAcm−2, open circuit voltage (Voc) of 638.3 mV and efficiency of 18.13% were fabricated. The result showed an improvement of about 13 mV on Voc compared to those of the reference solar cell fabricated with the reactive-ion etching back selective emitter and with Jsc of 36.90 mAcm−2, Voc of 625.7 mV, and efficiency of 17.60%.
The solar cell industry aims to produce high-efficiency solar cells at low cost. The industry has been able to reduce production costs by higher throughput and upscaling of the cell area. One way to reduce solar cell costs is to improve cell performance by applying cheap new methods .
Sheet resistance plays an important role in determining the efficiency of a crystalline silicon (C-Si) solar cell because it is related to the surface recombination velocity. The sheet resistance of a common solar cell for commercial applications is about 40 to 50 Ω/sq, which is achieved with homogeneous doping of the emitter region. This doping method can reduce the contact resistance in the metal–semiconductor interface. However, it would increase the surface recombination velocity, and thus, decrease the cell performance . The use of low sheet-resistant emitters in conventional crystalline silicon solar cells usually results in poor short wavelength responses . A lightly doped emitter would provide high sheet resistance and low surface recombination rate, resulting in high internal quantum efficiency in the short wavelength region. However, a lightly doped emitter has a high contact resistance and thus high series resistance . A heavily doped emitter has low contact resistance, but the lifetime of the generated carriers decreases due to the enhanced Auger recombination and Shockley-Read-Hall recombination .
To solve the problem of the trade-off between recombination and contact resistance, selective emitter solar cells are introduced. The emitter region where light generated carriers are collected is lightly doped to reduce the recombination velocity, and the emitter region below the contact is heavily doped to reduce the contact resistance .
The doping profile of the selectively patterned emitter has historically been obtained by using expensive photolithographic or screen printed alignment techniques and multiple high-temperature diffusion steps . Another way to obtain the doping profile of a selective emitter is to use an etching process such as laser, RIE, or wet etching. The RIE tends to damage the surface, and wet etching does not allow easy control of the sheet resistance .
In this paper, selective emitter solar cells are fabricated by the wet etching process. The process is optimized to improve the solar cell efficiency.
The rear side metallization was carried out with a standard aluminum paste by screen printing. The front contacts were formed by silver paste screen printing, followed by a firing step at low temperature of 150°C in a belt furnace for the metallization. Illuminated current–voltage (LIV) characteristics were measured under the global solar spectrum of AM1.5G at 25°C.
Result and discussion
Light current–voltage results of the reference, RIE etch back, and wet etch back cell
Fill Factor (percent)
Wet etch back
In the selective emitter solar cell, the sheet resistance around where the electrodes were to be formed was high, which reduces the contact resistance, thus reducing the cell series resistance. To realize a selective emitter, the high sheet resistance region around the electrode should be large enough. Figure 5c shows the drying conditions of the acid barrier paste after screen printing. The temperature and time were varied to find the conditions for properly hardened paste. In region A, the paste is not dried enough and cannot be used as the acid barrier. In region C, the pasted is too hardened and cannot be removed completely after the wet etching. Region B shows the optimal conditions for a selective emitter: drying temperature of 155°C for 70 min.
In this paper, we have presented a new wet etch back selective emitter method that uses the conventional etching paste used in screen printing to control the contact resistance. The HF and HNO3 (1:200) solution was used for 15 s to etch the acid barrier, which resulted in high sheet contact resistance of 90 Ω/sq. PC1D simulation was carried out to analyze the cause for the improvements in the cell characteristics of the selective emitter that underwent the wet chemical etch back process. Solar cells of 12.5 × 12.5 cm2 with a wet etch back selective emitter were fabricated, achieving an improvement of about 13 mV on the Voc compared to those of the reference solar cell fabricated with the RIE etch back selective emitter. The result showed, Jsc of 37 mAcm−2, Voc of 638.3 mV, and efficiency of 18.13%, for the cells fabricated with wet etch back; whereas Jsc of 36.90 mAcm−2, Voc of 625.7 mV, and efficiency of 17.60% were achieved for the RIE etch back. The wet etch back process gave more uniform and controllable contact resistance with less etching time than the RIE process, and hence, this process can be applied for mass production at a low cost.
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